In the past it has been well known to provide a loop antenna consisting of a wire rod or loop antenna consisting of a metal strip inside a casing of a radio apparatus or the like.
These small antennas, however, have low radiation resistance, and so have extremely low efficiency, and the directivity of these loop antennas depends upon the shape of the loop antenna. Still more, the gain of the loop antenna depends upon the area of the loop, which is under the limitation of the shape or size of the casing, so that it is difficult to make a radio apparatus more portable and smaller without reducing the gain of the antenna.